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DC Motor Drive Tripping Often
8

DC Motor Drive Tripping Often

DC Motor Drive Tripping Often

(OP)

Our client has a 440 V, 40 HP, 75A, 400 RPM base speed DC Motor driven by Eurotherm 590 + drive with 110 Amps capacity. The motor drives a rubber calendaring mill with a speed ranging from 400 RPM (field current 5.0 Amps) to 700 RPM (field current 2.4 Amps). The armature current as measured by an AC/DC clip on ammeter was found to vary from 20 Amps to 65 Amps. The current limit of the drive has been fixed at 95% of rated 75 Amps by the drive manufacturer’s rep.

This DC drive has tripped many times since its recent installation on “pulse missing”. In the field weakened range of about 650 to 690 RPM, the drive often goes to current-limit, the armature voltage is seen to vary from 200 to 570 Volts, and the field voltage from 70 to 270 volts. The drive indicates a speed error of 1.2 to 1.5%. At lower speeds, the armature and field voltages are stable.

1. What could be causing these trips on “pulse missing” ?

2. Is the current limit of 95% of rated current (as set by the drive rep) correct ? (Wouldn’t 125% be the norm for over current limits ?)

3. Why this hunting of armature and field voltages in the field weakened operation ?

4. The Eurotherm rep has advised to change the existing 4% line reactor to 2% reactor ? Is this ok ?

The same motor was previously working fine with a Siemens drive with 4% line reactor. The motor was also thoroughly checked after this tripping problem and found to be alright in all aspects.



RE: DC Motor Drive Tripping Often

Suggestion: The better rep will probably be the solution.
1. What could be causing these trips on “pulse missing” ?
///Improper timing. Relaxation oscillator, if present, is less accurate, i.e. fast or slow.\\\
2. Is the current limit of 95% of rated current (as set by the drive rep) correct ? (Wouldn’t 125% be the norm for over current limits ?)
///This may be set for sensitive protection of the motor, if the motor shaft is not fully loaded. However, the drive protection should be according to the drive spec sheet or manual.\\\
3. Why this hunting of armature and field voltages in the field weakened operation ?
///Apparently, the motor shaft load is bigger than the drive delivered energy.\\\
4. The Eurotherm rep has advised to change the existing 4% line reactor to 2% reactor ? Is this ok ?
///This may reduce the voltage drop across the reactor; however, the harmonics will increase on the drive input, if the reactor is on the drive input. If it is on the drive output, then the voltage drop will be reduced; however, the harmonic content will increase.\\\

RE: DC Motor Drive Tripping Often

I wonder if a rectifier device on one AC phase is not opening and closing properly.  This would cause the other phases to overload as they supply the DC load.  Could your drive alarm relate to the input side of the drive, rather than the output?

If can look at the DC waveform, you will see a periodic missing peak when one phase of the AC side is not carrying load.

What you may also find, on the AC side, is a current draw on only side side, i.e., negative or positive.  This would confirm a rectifier device fault.

I have encountered this type of fault on several occasions, so perhaps you have something like it.

RE: DC Motor Drive Tripping Often

Suggestion: A systematic analysis including waveforms monitoring as stated in the previous posting, might lead to the quick solution.

RE: DC Motor Drive Tripping Often

(OP)
Thx jb & testech. I will ask my client to check the waveforms as suggested.

RE: DC Motor Drive Tripping Often

Suggestion: Any diagnostics support from Eurotherm would also help (hopefully, it will come from a good tech support)

RE: DC Motor Drive Tripping Often

I have exactly the same problem with the same drive on a much larger system (I have two 300hp Drives) I have tried everything, adding line reactors load reactors, monitoring the supply line with chart recorders the works to avail. It still is a problem today. The customer just has to run the machine slower to stop it from tripping.
I am certain that the problem is at component level on the drive my next move is to get a Eurotherm guy in to strip it down and start swopping out boards.

RE: DC Motor Drive Tripping Often

(OP)
snoogie,

My client just informed me that Euortherm rep disabled the "Pulse Missing" trip. Apparently, he required some password from the HQ in London (?) to do this. Still, he could not explain why the trip occurs in the first place. May be your Eurotherm guy can do better. If so, pls share it in the forum.

RE: DC Motor Drive Tripping Often

edison,

I believe the Eurotherm HQ is in Littlehampton on the south coast of England.

RE: DC Motor Drive Tripping Often

(OP)
Thx scotty.

RE: DC Motor Drive Tripping Often

Edison:
Do you know what pulse missing means?

Did you obtain current or waveforms to ensure that rectification is occuring properly.  You can also obtain vibration readings.  Improper rectification can result in high vibration at 10,800 or more likely 21,600 CPM.   

I just had a case where a DC motor was reportedly producing a large magnetic field that interefered with instrumentation.  My analysis revealed an increase in 21,6000 CPM vibration from .05"/sec to .41"/sec.  The DC current waveform revealed extreme peaks of extremely short duration, rather than half cycle duration for each ripple. (Too bad I can't attach the waveforms here, they are very interesting.) In addition,  one ripple was actually negative going, suggesting the motor was enduring reverse torque.  They had to slow the motor down to keep it on line.

They are now having the drive manufacturers look for a timing or other fault in the drive.  The SCR's or whatever is being used in this drive for rectification is clearly not staying open long enough and in the proper sequence.  

The bottom line here is that you can learn a lot about the drive operation from both the vibration signatures and the current signature.

RE: DC Motor Drive Tripping Often

(OP)
testtech,

The motor vibration levels at various speeds and loads do not exceed 0.2 mm/sec. By hand, you can feel the motor running very smoothly. Maximum motor temp is only 58 deg C.

Have you had any such expereinces with the drive mentioned ?

RE: DC Motor Drive Tripping Often

edison:

That vibration level is so low, you have got to wonder if the machine is running!

In the past, I had a drive that ran heating elements at a computer chip plant.   The drive continuously  went out on overload.  Examination of the input and output waveforms revealed a "pulse missing" in the sense that AC on one phase was not drawn on half a cycle.  The resulting waveforms had a "pulse missing".  The drive tried to make up the load by increasing the load on the other two phases.  This increased the loads sufficiently to trip the drive.  

If you go to page 7-12 in the Eurotherm manual, http://www.eurothermdrives.com/doc/HA470388.pdf
it describes a missing pulse.  This condition means that one of the six pulses is missing from the the armature waveform and you probably have a defective SCR or SCR control.  This is the condition I was referring to on my first reply to this thread.

Once again, check the DC current waveform using an o-scope.

RE: DC Motor Drive Tripping Often

Edison:

Another commment.  Since the vibration level is so low, you may not actually have a waveform fault.  The page cited above also lists control related faults that may occur even if the waveform is properly formed.  It seems that these control related functions might be the place to look.  But, the easy starting point is checking the waveform.

RE: DC Motor Drive Tripping Often

(OP)
Thx testtech.

RE: DC Motor Drive Tripping Often

Does this particular drive have a setting for the front limit firing angle?  I have worked with some drives where the firing angle is determined from a separate sync supply where a saftey margin was required when setting this limit.  The drive could be trying to fire before the phase crossover point.  The field controller may be hunting due to armature reaction when the drive goes into current limit.

RE: DC Motor Drive Tripping Often

This dosent look like a bad scr or gate circuit problem.If you had a problem with these your currents would run high at
all modes of operation, not just in field weakening.
The pulse missing fault is most likely caused by some feed-
back element.This raises the question of how the drive gets
speed feedback;tachometer,encoder, or "armature" feedback.
Also could you describe what type of current feed back your system uses.Also I assume you are using a speed control mode of operation as opposed to constant current.
The fault could also be in your "zero crossing" circuit.
This citcuit lets the gate curcuit know when the observed phase is rising from zero volts so the gate can be fired at the calculated angle of the cycle. You could check this with
a dual trace scope connected at the zero crossing test point and the input of the phase to the drive for problems with sync pulses on the z.c. side not matching with the phase zero crossing.
You current limit is set to high at 95% rating of the drive.
You "current limit" should never be set above moter rated full load. Forget about inrush as if you were sizing a curcuit breaker.
I think the hunting of voltages in your armmature and field circuit is normal. Your drive is just tring to keep the speed of you motor constant under demanding conditions.
I dont think the reactor change will help this problem.
Overall though this sounds like poor drive tuning.


RE: DC Motor Drive Tripping Often

Hi, it maty be that the armature voltage is set too high on the field controller. If the arm voltage rises to above the peak incoming main, the thyristor does not get forward biased and so does not fire.

RE: DC Motor Drive Tripping Often

We have had this happen to us on a 590+ drive system. 480v 30hp. What we found was interference on the feedback and on the signal wire. Solution: Run shielded cable to the motor and reduce the feedback to the drive.

RE: DC Motor Drive Tripping Often

(OP)
Thx barac, electromike, cbarn & buzzmore for your views and time. I will certainly pass your ideas & suggestions to my client.

electromike,

The current limit is set at 95% of motor current of 75 Amps. The drive is rated for 110 Amps. I think the current limit should be set to atleast 110% of motor current to avoid frequenct "Current Limit" operations. Do you agree ?

RE: DC Motor Drive Tripping Often

Suggestion: Rather than to measure the motor by ammeter (obtained values from 20A to 65A stated in the original posting), it will better to record the current. 95% setting of 75A gives 71.25A. Therefore, the motor shaft load could easily cause to exceed 71.25A since there are other factors to take into consideration, e.g. voltage fluctuation, manufacturing tolerances of motor drive, flaws in the motor drive, etc.

RE: DC Motor Drive Tripping Often

The current limit setting is dependant on your motor design and application.  When I worked in a steel mill our engilsh electric 'mill motors' had a current limit of 2 per unit.  Since the application required very quick screwdown movements and as they were not continous the overall motor heating effect was ok.  The important point to consider is not to exceed the i2t of the motor or drive.

RE: DC Motor Drive Tripping Often

Edison, the fluctuations in both current and voltage are highly unusual, even if the pulses are misfiring. Is the field weaking loop tuned properly?  Noise on the tach, as mentioned by buzzmoore could be a contributing factor.  Monitor the output of the speed regulator with a scope. I assume it is quite eradic.  For troubleshooting purposes, the drive can be detuned to see if this has any affect.

To anyone else with a missing pulse, if it is possible, can you monitor the current feedback waveform?  Preferably with a storage scope.  I would like to know exactly how many pulses are missing, if in fact they are missing.

Also, make sure all of the gate leads which go to the Scr's are secure. I have seen in one case where this was a problem on a drive mounted in a high vibration area.

 

RE: DC Motor Drive Tripping Often

Testtech,
   Just in case you are not aware of this, if one Scr is open or not firing, for example, you will not be missing 1 pulse on a 3phase, 6 pulse system.  You will, in fact, be missing two consecutive pulses.  
 

RE: DC Motor Drive Tripping Often

edison125,

 Your operating current limit should be a percentage of  the maximum current of the drive. Which in this case is 110A. So at 95% of 110A your drive will allow your motor to run at 104.5A. Most drives have a parameter that limits the time a drive will run in limit. The term current limit also seems to be getting confused with what should be the current trip of the drive. Some drives allow you some range in setting current trip. Keep in mind when you hit current trip your drive shuts down instantly.(At least your drive stops instantly, your conducting scrs will try to carry the high current until the holding current drops below the given value for the scrs. Not good.) The reason for the instant shut down is to protect your scrs,,semi-fuses, and load. This is the way the systems I know operate and I suspect this one is no different. You can exceed the full load current for a very short time without heat and commutation problems in D.C. motors. I would check armature current when the drive is in limit to investigate what the current is realy doing.

RE: DC Motor Drive Tripping Often

Comment: It is not clear from the original posting how old is the DC motor drive. If the drive is aging, the capacitor values are adrift and so are resistor values. This may cause various irregularities in the electronic triggering circuits.
Also, the ambient temperature is not mentioned in the original posting. The high ambient temperature can cause electronic circuits to have changed operating points and quiescent points, which can have detrimental impact on the gate triggered switching devices.

RE: DC Motor Drive Tripping Often

(OP)
Thx everyone. I passed your views to my client. Eurotherm rep by bypaased the "Pulse Missing" signal after getting some special password from UK and the current limit has been set at 100% of rated motor current. The drive is working fine now.

Thx again for your time and views.

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